1. Field of the Invention
The present invention concerns a method to excite nuclear spins in an excitation in an examination subject so as to acquire magnetic resonance signals from a region of the examination subject, as well as a magnetic resonance system for implementation of the method.
2. Description of the Prior Art
Magnetic resonance tomography is a method in widespread use for acquiring medical image data of a subject to be examined. The subject to be examined is brought into an optimally homogeneous static magnetic field (B0 field). To acquire image data, magnetic gradient fields are applied and electromagnetic radio-frequency (RF) pulses are radiated that excite nuclear spins processing around the magnetic fields. The magnetic flux density of the RF pulses is often designated with B1. Given an excitation with RF pulses, the direction of the magnetization (which is originally aligned parallel to the B0 field) is tilted by a predetermined angle (flip angle) relative thereto. The flip angle depends both on the radiation duration of the RF pulse and on the B1 field strength. For example, given a flip angle of 90°, a transverse magnetization (perpendicular to the B0 field) can be generated that exhibits a decay that is subsequently acquired as a magnetic resonance signal. Consequently, to generate qualitatively high-grade image data, it is desirable to obtain an optimally uniform deflection of the magnetization across the slice of the subject to be examined.
The transverse magnetization decays by means of spin-spin relaxation during a decay time or relaxation time T2. The decay of the transversal magnetization with T2, however, occurs only in an ideal homogeneous magnetic field. In a real magnetic field, the transverse magnetization decays with the time constant T2* due a certain degree of inhomogeneity of the real magnetic field. The T2* relaxation time is normally significantly shorter than the T2 relaxation time. Nevertheless, in order to obtain a signal that is dominated by T2, spin echo methods are used, wherein a 180° refocusing pulse slews the transverse magnetization. The transversal magnetization consequently re-phases and generates a spin echo signal.
Different T2 relaxation times for different tissue or fluids can be used in order to establish a contrast between these signal sources. For example, a fluid with longer T2 relaxation time is imaged with high contrast in magnetic resonance cholangiopancreaticography (MRCP). In order to acquire a high signal of the fluid, strongly T2-weighted multi-echo sequences are used, for example turbo spin echo sequences (TSE). These TSE sequences conventionally include a number of 180° refocusing pulses with constant, high refocusing flip angles in order to cause signals to be acquired essentially only from regions of an examined person that exhibit long T2 relaxation times. The conventional MRCP imaging method consequently results in a high energy injection into the examined person. Since signals with short T2 times are also acquired at the beginning of the echo sequence, background signals, that are caused by blood and fat and can interfere with a diagnosis, often remain in the image data. The use of selected radio-frequency (RF) pulses for fat saturation also leads to an incomplete suppression of signals of body fat due to B0 and B1 inhomogeneities. The B1 amplitude additionally varies spatially, particularly in a high field, which leads to spatially-dependent, inhomogeneous signal intensities across the field of view. Due to the necessary use of a pulse sequence with high flip angles, a high power of the magnetic resonance system is furthermore required for MRCP imaging. Disadvantages of a conventional MRCP are thus a high energy injection into human tissue, an incomplete suppression of background signals, and the influence of B1 inhomogeneities.
Due to the strong T2 weighting, an MRCP imaging requires many refocusing pulses with large flip angles. In order to reduce the energy injection, the number of refocusing pulses could be reduced or the refocusing flip angles could be reduced. However, these measures reduce the acquisition efficiency of magnetic resonance signals and the signal intensity acquired from a fluid.
In order to reduce the background signals that are caused by blood and fat, it was proposed by Busse et al. in “Improved Background Suppression for 3D-MRCP using T2-Prep”, Proc. Intl. Soc. Mag. Reson. Med. 14 (2006), Page 392, to delay a data acquisition after an excitation or to implement a separate, short T2 preparation before an excitation. However, this method does not solve the problem of the high energy injection, since a large number of refocusing pulses with large flip angles is required.